As Sound Transit begins the next phase of extending light-rail service to Northgate, a retired architecture professor is calling on the agency to build a public plaza above the future U District Station, a stop that will accommodate some 12,000 riders a day.

But transit managers have their eye on a development deal with the University of Washington, which could add housing and retail at the station site — a boon for “transit-oriented development.”

The debate over development of the station is emerging even as politicians on Friday held a ceremonial groundbreaking for the $2.1 billion, 4.3-mile Northgate Link.

The line would open for service in 2021, extending light rail from Husky Stadium to the University District, and then north to the Roosevelt neighborhood before it reaches Northgate.

The U District Station site — south of the Neptune Theatre on Brooklyn Avenue Northeast between about Northeast 43rd and 45th streets — is uniquely located for a public square, says Philip Thiel, a UW professor emeritus of architecture.

His model shows a brick piazza three-quarters of a block long, with a new east-west pedestrian walkway connecting the station stop to University Way Northeast.

Across Brooklyn there already is a 22-story UW office building (known as the UW Tower), complete with a sculpture by Seattle native George Tsutakawa.

The station’s “dimensions are almost exactly that of a European square,” Thiel said, favorable for farmers markets, street fairs and outdoor cafes.

The block is now zoned for buildings up to 65 feet in height, and on Thursday, the transit board is scheduled to consider declaring air rights above the future underground station as surplus — allowing staff to make a deal with the UW.

University spokesman Norm Arkans said the UW hasn’t chosen a specific project yet, and is also talking to plaza supporters. A longstanding UW goal has been to make the area more livable by attracting residents.

“We’d like to get more people back there,” Arkans said. “We’d love to have more capability for faculty and staff to live closer to work.”

The university owns one of two land parcels at the station site.

Former Seattle City Councilmember Peter Steinbrueck is assisting Thiel. They say the transit-friendly high-rises ought to go on other lots nearby.